Radar Structure of Earthquake-Induced, Coastal Landslides in Anchorage, Alaska
WALTER A. BARNHARDT and ROBERT E. KAYENU.S. Geological Survey, 345 Middlefield Rd., Mail Stop 999, Menlo Park, CA 94025
© 2000, AAPG/DEG, 1075-9565/00/$15.00/0
Key Words: earthquake hazards, ground-penetrating radar, landslides.
The city is underlain by up to 180 m of late Quaternary deposits that are heterogenous in texture (Miller and Dobrovolny, 1959). The stratigraphy generally consists of (1) glacial till unconformably overlying bedrock, (2) silty clay of the Bootlegger Cove Formation resting on till or bedrock, and (3) outwash sand and gravel on the surface (Miller and Dobrovolny, 1959). Till locally crops out in a large moraine complex that lies just north of the city, marking the most recent advance of late Pleistocene glaciers into the area. The finely laminated silty clays of the Bootlegger Cove Formation were deposited in front of the glaciers in a glacialmarine to glacial-lacustrine setting. These fine-grained deposits average 30-45 m thick in the vicinity of downtown Anchorage and contain discontinuous layers of sand and scattered pebbles (Seed and Wilson, 1967); water content is often at or near saturation. The surficial outwash is thickest (up to 18 m) near the moraine and thins toward the south and west, forming a wedge-shaped deposit of sand and gravel. In undisturbed sections, the contact between outwash and the underlying clay is nearly horizontal. The different properties of the sediment greatly influenced the distribution and magnitude of earthquake damage in 1964 (Hansen, 1965).
Earthquake-induced landslides in Anchorage occur in a variety of settings and exhibit different forms. The most damaging landslides primarily moved by translation rather than rotation and slid laterally along subhorizontal surfaces (Hansen, 1965). The failures developed within the Bootlegger Cove Formation, not in the overlying outwash deposits (Hansen, 1965; Seed and Wilson, 1967). Extensive geotechnical investigations of this formation by Shannon and Wilson, Inc. (1964) detected a central weak zone located between upper and lower zones of stiff, competent clay. Loss of strength in the sensitive central zone resulted from the fabric collapse of quick clay during the long duration of ground shaking. Liquefaction of sand layers and lenses within the clay deposits might have partially contributed to the failure of the bluffs (Seed, 1968). Stratigraphic sections (Figure 2) were produced shortly after the earthquake and helped elucidate the mechanics of these dramatic bluff failures (Hansen, 1965; Seed and Wilson, 1967).
Common midpoint (CMP) surveys also were performed at each site to determine the velocity of radar through the ground and thus better constrain the depth of subsurface features. Whereas a reflection survey bounces radar energy off a series of points along a reflector, all shots in a CMP survey are centered on a single point on that same reflector (Figure 3). The antenna separation is not constant, rather the transmitter and receiver are moved in opposite directions; the distance between them increases by a set increment with each shot. By expanding the horizontal separation symmetrically about a central point, the system measures the travel times along multiple paths through the same geologic unit. The velocity analysis assumes a constant velocity model for the Earth, where the travel times for signals along these paths varies in a hyperbolic manner. After adjustments for normal moveout (i.e., changes in path length), the CMP traces are stacked or added together along hyperbolae of many different velocities. Where the modeled velocities are too fast or too slow, the traces do not stack coherently. At the correct velocity, however, the signals do stack coherently and thus are amplified. By plotting and visually picking the highest amplitude signal, we determine the best-fit velocity for a given unit. This estimated velocity may provide insight into the type of sediment present, and when applied to reflection profiles, it allows the conversion from travel time to depth.
Approximately 400 m of GPR reflection profiles were collected at the Government Hill site. The profiles started on the bluff top and crossed the slide from north to south in the general direction of slippage. A strong, continuous reflection is visible in the subsurface and mimics the shape of the ground surface (Figure 5). This reflection is interpreted as the upper contact of the clay-rich Bootlegger Cove Formation, which is buried beneath outwash sand and gravel. Abrupt changes or offsets in the depth of the reflection may represent normal faults along the margins of the graben. The profile descends ~7 m down the steeply sloping scarp to the flat floor of the innermost graben. Outwash deposits in the graben are 3-4 m thinner than in the adjacent undisturbed section. Narrow, wedge-shaped packages of strong GPR reflections in the graben floor probably represent the parallel series of ground cracks or crevasses that appear in photographs from 1964 (Figure 4). We believe that natural deposition or landscaping filled the cracks with loosely packed sediment, whose bulk density differs from surrounding landslide debris and thus creates the anomalous, wedge-shaped packages of GPR reflections. A displaced, flat-topped block (horst) on the south side of the graben exhibits little stratigraphic disturbance and no ground cracks. The GPR profile crosses over the block and down into a second graben that opened on the south side (Figure 5). Note the disruption of the originally horizontal outwash/clay contact beneath the scarp and graben floor. Extensive regrading on this part of the slide has removed a second displaced block that is visible in 1964 photographs (Figure 4).
In conjunction with the reflection profiles, a CMP survey also was performed adjacent to the Government Hill landslide. In this area of the original bluff, ~15 m north of the main scarp, the stratigraphic section is relatively undisturbed and experienced no significant ground cracking during the earthquake (Hansen, 1965). Based on data from the CMP soundings (Figure 6A), we estimate an average velocity of 0.12 m/ns through the upper unit of sand and gravel (Figure 6B). Given ~200 m of two-way travel time, we calculate a depth of 12 m to the prominent reflection in Figure 5. This value is in agreement with direct measurements made by Hansen (1965) just after the earthquake (Figure 2A). No additional reflections occur deeper in the section; thus, no velocity estimates are available for the underlying clay. The best method to measure velocity in the clayey sediment is crosshole GPR, a technique in which GPR antennas are lowered down a pair of closely spaced boreholes (e.g., Barnhardt et al., 1999).
Approximately 600 m of GPR reflection profiles were collected on the eastern part of the Turnagain Heights landslide. Three profiles were oriented north-south, parallel to the general direction of slippage, and crossed the steep, 10-m-high scarp at the head of the slide. In areas above the main scarp, GPR images reveal high amplitude, horizontal reflections at a depth of ~5 m below the surface (Figure 8). The flat-lying reflections represent the contact between coarse-grained outwash and the underlying glacial-marine clay and are only visible beneath undisturbed parts of the bluff, adjacent to the landslide zone. No clear outwash/clay contact appears seaward of the scarp, rather the slide deposits are internally chaotic. The absence of easily recognizable stratigraphy within the slide volume is probably the result of disintegration of the sediment mass during sliding. Unlike the relatively simple horst and graben structures at Government Hill, the slide at Turnagain Heights created "hundreds of sharp crested clay ridges alternating with collapsed troughs. . . (that) ranged in height from about 10 to 15 ft (3.0-4.5 m)" (Hansen, 1965 p. A61)." The rugged topography of the slide deposits has experienced only minor modification by erosion, and a large area is preserved in the state it was in 1964. By necessity, fieldwork was confined to trails and roads due to impassable willow thickets and many water-filled depressions. The profile in Figure 8 was collected along an established access trail, where the ground surface has been modified.
A CMP survey also was performed at Turnagain Heights on top of the original bluff. Local ground cracks developed at this location during the 1964 earthquake but the stratigraphy remains relatively intact (Hansen, 1965). Based on the CMP data, we calculated a velocity of 0.10 m/nsec for the upper unit of sand and gravel. By using this velocity, the prominent reflector at 100 ns (two-way travel time) represents a depth of 5 m (Figure 8), approximately the same thickness as observed in cores and outcrops (Figure 2; Seed and Wilson, 1967). Although the velocity is slower than through comparable material at Government Hill, it is well within the range of empirically derived values for coarse-grained sediment. The lower velocity at Turnagain Heights may indicate higher water content, an important factor that may partially explain the catastrophic nature of the landslide.
At Government Hill, the inertial-displacements resulted in an observed morphology of laterally displaced glide-blocks (Figures 2A, 4, and 5). The blocks consist of outwash deposits and part of the underlying clay deposits, which were rafted as a largely intact crust atop a softer deforming zone. There is little internal deformation of the blocks themselves, which lie between and adjacent to large graben. A house on one block moved laterally ~10 m during the quake (Hansen, 1965), but suffered little damage and remains occupied today. Morphology of the landslide suggests that the residual shear strength of the deforming clay unit exceeded the gravitational shear stress induced on the slide mass. That is, it appears that after earthquake motions ceased, the blocks did not continue to accelerate downslope and disintegrate. The sliding at Government Hill was certainly dramatic and damaging but not characteristic of a disintegrative-flow morphology. Varnes (1978) classifies this relatively simple type of landslide as an earth block glide.
The morphology and magnitude of deformations at the Turnagain Heights landslide characterize it as an earth lateral spread (Varnes, 1978). At this site, the internal structure of slide deposits is chaotic, and the original stratigraphy is unrecognizable in GPR profiles (Figure 8). The bluff apparently lacked substantial buttressing to resist horizontal shear stresses (Wilson, 1967), and observers noted that lateral sliding toward Knik Arm continued long after the shaking stopped (Seed and Wilson, 1967). The ultimate residual-shear resistance apparently fell below the downslope gravitational-shear stress, such that the mass continued to deform after inertial loading ceased. These continued motions are indicative of disintegrative-flow behavior, the state in which the residual-shear resistance is less than the slope stress. Once the slide mass was mobilized, gravitational-shear stress alone exceeded the strength of the remolded clay, leading to very large deformations and even catastrophic flow-sliding. Certainly, at Turnagain Heights the resultant morphology and incoherent stratigraphy of the landslide deposits is indicative of just such an unlimited and highly destructive flow slide.
Most of the two sites are preserved as city parks today, with landscapes that remain relatively unmodified since the 1964 earthquake. Future earthquakes may cause additional movement along these sections of bluff, but few occupied structures will be at risk. Despite the potentially unstable soils, however, development on adjacent areas has continued since 1964. A mix of private housing, military facilities, and fuel storage tanks are built on or near old landslides that line the margin of Government Hill (Varnes, 1969; Updike and Carpenter, 1986). The City of Anchorage initially prohibited development at the Turnagain Heights site, but new home construction has resumed and the city has spent >$2 million to build roads, sewer, and water lines across the landslide debris (Doogan, 1998). Although inclonometer analyses from 1965-1980 detected negligible strain in the Turnagain Heights area (Updike, 1983), evaluations of slope stability indicate that further development should proceed with caution (Updike et al., 1988).
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Robert E. Kayen
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last modified 1 December 2003 (lzt)
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